Suction Filtration

Crystals may be collected from a solution by filtration on a Büchner funnel. A partial vacuum is created in the filter flask by a water aspirator. (Water flowing past a small hole in a pipe draws in gas from the hose to the filter flask. This reduces the pressure creating the partial vacuum. Additional information on the operation of an aspirator can be obtained by checking a physics textbook under "Bernoulli's principle".) The filter paper should be placed in the funnel and wetted before starting the filtration to help seal it to the funnel. Ensure that the filter flask is clamped to the retort stand and the rubber stopper holding the Büchner funnel is tight in the filtration flask. Turn on the water to the aspirator to create a reduced pressure in the filtration flask. Transfer the solution and the crystals into the funnel - be careful not to overflow the funnel! Swirl the last of the solution in the flask to help transfer any remaining crystals into the funnel. Use a spatula to transfer crystals that are missed. Wash the crystals as instructed in the lab manual and then compact the crystals with a spatula to push out as much solvent as possible. Leave the crystals on the Büchner funnel with the aspirator running to further dry the crystals. Check the lab manual for any drying instructions.